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Re: [PATCH] [Aarch64] Optimize subtract in shift counts


Richard Biener <richard.guenther@gmail.com> writes:
> On Tue, Aug 8, 2017 at 10:20 PM, Richard Kenner
> <kenner@vlsi1.ultra.nyu.edu> wrote:
>>> Correct. It is truncated for integer shift, but not simd shift
>>> instructions. We generate a pattern in the split that only generates
>>> the integer shift instructions.
>>
>> That's unfortunate, because it would be nice to do this in simplify_rtx,
>> since it's machine-independent, but that has to be conditioned on
>> SHIFT_COUNT_TRUNCATED, so you wouldn't get the benefit of it.
>
> SHIFT_COUNT_TRUNCATED should go ... you should express this in
> the patterns, like for example with
>
> (define_insn ashlSI3
>   [(set (match_operand 0 "")
>          (ashl:SI (match_operand ... )
>                      (subreg:QI (match_operand:SI ...)))]
>
> or an explicit and:SI and combine / simplify_rtx should apply the magic
> optimization we expect.

The problem with the explicit AND is that you'd end up with either
an AND of two constants for constant shifts, or with two separate patterns,
one for constant shifts and one for variable shifts.  (And the problem in
theory with two patterns is that it reduces the RA's freedom, although in
practice I guess we'd always want a constant shift where possible for
cost reasons, and so the RA would never need to replace pseudos with
constants itself.)

I think all useful instances of this optimisation will be exposed by
the gimple optimisers, so maybe expand could to do it based on
TARGET_SHIFT_TRUNCATION_MASK?  That describes the optab rather than
the rtx code and it does take the mode into account.

Thanks,
Richard


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